STUPID-2026-0074

Copilot CLI destroyed its own 233MB session log trying to "back it up" with a hardlink instead of a copy

5.0medium
June 24, 2026Source checkedRepro reported

Why 5/10

The agent destroyed its own 233 MB raw session log. The session itself, and the user's work, survived.

Scope
0/4
Time, tokens or the agent's own working state only
Recoverability
3/3
Permanently lost or irreversible, including data that was exposed
Did it happen
2/2
Happened in real use
Beyond the user
0/1
Only the user's own non-production work

The score is the sum of these four factors. How each factor is scored · scored 2026-10-29

  1. Instruction given

    No explicit instruction for this step — the agent was working autonomously within a long-running (~1200 turn) Copilot CLI session and decided on its own to truncate its bloated events.jsonl log to relieve a CPU-pinning issue it was also trying to diagnose, first attempting to preserve the old contents as a backup.

  2. Expected behavior

    Before truncating a large log file "as a backup," make an independent copy of it (e.g. with `cp`), not a hardlink — a hardlink shares the same inode as the original, so truncating either name truncates both.

  3. Actual behavior

    The agent ran `ln events.jsonl events.jsonl.bak-20260624` followed by `: > events.jsonl` to truncate the live log after "backing it up." Because `ln` creates a hardlink rather than an independent copy, both the original file and the supposed backup shared the same inode, so truncating one truncated both simultaneously.

  4. Damage

    The full 233MB raw events.jsonl transcript for the session was destroyed with no way to recover it, despite the agent's own belief it had made a backup first. The session itself was not lost — plan.md, checkpoints/, session.db, files/, and stored memories were untouched — but the raw event history was gone. The same report also flagged that this file's uncapped growth was independently pinning ~5 CPU cores for 20+ minutes on session resume.

Full write-up

While troubleshooting a separate performance problem in a long-running Copilot CLI session — an events.jsonl log that had grown to 233MB over roughly 1,200 turns and was pinning multiple CPU cores on resume — the agent tried to shrink the file safely by making a backup first. It ran ln events.jsonl events.jsonl.bak-20260624 to create what it treated as a backup copy, then truncated the original with : > events.jsonl. A hardlink doesn't create an independent copy; it creates a second name pointing at the same inode, so truncating the "original" truncated the "backup" right along with it. The entire 233MB raw transcript was destroyed in the process the agent had chosen specifically to preserve it. The reporter, who filed the issue against GitHub's own copilot-cli repository, noted the session survived because its plan file, checkpoints, session database, and stored memories lived elsewhere — but the full event history did not, and there was no way to get it back. The filer suggested the project either document that ln is not a backup primitive (recommending cp instead) or ship a proper /session-archive command so agents don't need to improvise file-level backup tricks on a live session.

Classification

Agent
GitHub Copilot
Failure mode
Destructive Action
Root cause
Tool Misuse
Domain
Data
OWASP Agentic Top 10 (our mapping)
ASI02 Tool Misuse and Exploitation list
MIT AI Risk Repository (our mapping)
7.3 Lack of capability or robustness taxonomy
Source
Github Issue

Related incidents

7.0
Copilot CLI's checkpoint restore ran git clean -fd and destroyed 1GB of evaluation data the agent never touched
9.0
OpenAI's own cybersecurity-evaluation agents broke out of their sandbox, chained zero-days, and compromised Hugging Face's production infrastructure trying to win an impossible benchmark
7.0
Claude Code moved files into a log/ subfolder, then rm -rf'd the parent directory containing it — 1,500 files gone (GitHub #49129)
7.0
Claude Code in auto mode ran an unrequested Remove-Item -Recurse -Force and permanently deleted two days of research after a source/destination name collision during a copy task (GitHub #94453)
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Cite this record
StupidLLM. "STUPID-2026-0074: Copilot CLI destroyed its own 233MB session log trying to "back it up" with a hardlink instead of a copy." AI Agent Incident Database, 2026. https://www.stupidllm.com/incident/STUPID-2026-0074/ (accessed 2026-11-11). CC BY 4.0.

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